The present disclosure is directed to a pressure relief valve installed with a mud pump. The function and operation of this valve will become more apparent on a review of the context in which the device is used. To set the stage, consider the operation of a typical triplex mud pump which is delivering a large volume of mud flow for a drilling rig. The mud is delivered to the drill stem to flow down the string of drill pipe and out through the drill bit appended to the lower end of the drill stem. It flows through the drill bit. The flow of mud cools the drill bit and reduces the temperature so that it lasts longer. Moreover, the mud flow is jetted out through a set of openings in the drill bit so that the mud hydraulically washes away the face of the well borehole if it is formed of soft materials. In addition, it washes away rock chips and cuttings which are generated as the drill bit advances. Then, the mud flow must return to the surface in the annular space on the outside of the drill stem and on the interior of the open hole formed by the drilling process. While portions of the borehole may be cased from the surface, the mud flow must be of sufficient velocity that the mud is returned to the surface so that chips and cuttings which are inherently heavier than the mud are flushed to the surface and delivered. This requires a substantial flow velocity. The cooling necessary also requires a substantial velocity. The mud flow velocities required mandate a high volume of mud. It is not uncommon that a triplex mud pump will deliver 500 or even 1,000 gallons per minute through the drill stem. This substantial flow must be delivered under control. As will be understood, the flow path down through the drill stem and back through the annular space describes a U-tube. The U-tube will therefore prompt a return at the surface but not with adequate pressure. Pressure levels at the pump side have to be raised to get the velocity and volumetric throughput desired. It is not uncommon for the triplex pump to be required to deliver mud flow at 1,000 psi and even higher. The wellhead pressures at the pump must be much higher if there is substantial flow pressure resistance along the flow path. The pump therefore is often operated at a very high pressure.
The drill stem in a deep well is an impediment to flow, thereby resulting in higher back pressures. The impediment to flow is overcome by applying greater pressures at the surface. In this regard, the mud pump typically could be operated at pressures as high as 5,000 psi output. Because of the great variety of circumstances in which the drilling rig may be used, the output pressure of the mud pump may vary widely. In one regard, the mud pump output pressure will vary with the change in pump speed. Sometimes, the prime mover for the mud pump will not run smoothly. The mud pump itself has a characteristic pressure peak signature. Normally, mud pumps are constructed with three large cylinders which provide three pressure peaks during each cycle of operation. These pressure peaks can be excursions as great as 200 or 300 psi on top of the prevailing baseline pressure.
Because of variations in motor speed, because of the three cylinder construction involved, the mud pump output pressure will vary significantly. The present disclosure is directed to a regulator which is a pressure relief valve able two control and stabilize the pressure downstream of the triplex mud pump. The mud pump output manifold is input to the pressure relief valve of the present disclosure. That valve is operated so that the output pressure is controlled to a desired level. The output pressure accomplished by the present regulator enables the system to operate with a controlled pressure level. This is important to insure that the pressure experienced down hole at the drill bit and in the formations penetrated by the drill bit is regulated. Mud pressure at the bottom of the drill stem is an important factor. The pressure has to be maintained at a certain range to prevent formation damage. As the well is drilled, there is a tendency for the liquids in the mud to migrate into the formation and to form a residue known as a mud cake in the well borehole. The mud cake is generally desirable. The depth of penetration into the formation by the liquid solvent is not as desirable in that it may force the hydrocarbons in the formation back away from the well borehole and make it more difficult to start production flowing. That is one problem with excessive bottom hole pressure. Where the bottom hole pressure is inadequate, there is the risk that gas flow will start, and the gas will then cut or thin the drilling mud. Typically this happens with gas bubbles entrained in the mud which are small in size at the bottom of the borehole but which become larger at the surface where the hydrostatic head on the mud is less. This thinning of the drilling fluid describes a process known as gas cutting. When the mud is gas cut, it is not as reliable in operation because the standing column of mud in the well borehole does not have the necessary weight. In other words, it tends to froth and reduces the bottom hole pressure, creating a dangerous condition. The bottom hole pressure is in part controlled by the mud pump pressure. It is therefore rather important to regulate and control the mud pump pressure.
The apparatus of this disclosure is briefly summarized as a pressure regulator having a movable element which is dynamically responsive to pump output pressure. The movable element is located in the mud flow path and is installed so that it modulates the mud flow. Dynamically, movement is occasioned by the movable element which is in a slight pressure balanced condition. On one side, pump pressure is applied. On the other side, the same pump pressure is applied but the second side is less in diameter and has a reduced cross-sectional area. There is a compensation applied to this side for balance purposes. That compensation is provided by a piston rod extending to a remote chamber, and the chamber itself is provided with a hydraulic fluid pressure balance across the chamber. Through the use of a restricted flow path, movement of the rod is controlled so that it modulates the forces applied to the valve element. This accomplishes a desired setting under the control of hydraulic fluid. That is provided with a reduced hydraulic pressure. There is a multiplier which reduces the hydraulic pressure required for an operating range.